Optical fibers can be used to transmit power. To transmit along a fiber, electrical power is first converted into light with a power conversion device, such as a multimode pump chip. The high power light is directed into the fiber at a fiber tip, and then travels down the fiber to a destination, or is coupled into another fiber. To achieve optimal coupling at the fiber tip, the fiber tip is accurately aligned with the light source, and, once aligned, securely held in place. A typical securing technique involves stripping a 10-20 mm length of the jacket off the fiber at one end, metallizing the exposed glass of the fiber, inserting the fiber through a mounting tube, and securing the fiber to the mounting tube. The mounting tube is then secured to a mounting block.
High-powered optical fibers have been secured to a mounting tube with a metallic solder applied to the metallized surface of the fiber. When light traveling inside the fiber reaches the fiber wall, a significant portion of the light is deflected out into the metal. The deflected light is rapidly absorbed since the metallized surface of the fiber, as well as the solder, do not transmit light. Furthermore, the interface between the fiber and the solder contains a complex web of oxides and other dielectric materials that also absorb light. Modem multimode power-carrying optical fibers typically carry a total power of about 10 watts. Since about 10% of the fiber's power can be coupled to the metal layer and solder, this coupling can result in the deposition of about one watt within a few millimeters around the solder junction. Such energy deposition can cause intense localized heating, which can cause the solder to melt, and thus cause serious damage to the fiber and the surrounding components.
In one alternative approach, the metallic solder is replaced with glass solder or with epoxy. However, because these materials have a refractive index that is similar to that of glass or even a little higher, they refract light out of the fiber, also causing power loss. Oxides within the glass solder are efficient light absorbers, and the result can again be significant localized heating with potentially destructive consequences.